The invention relates generally to the field of computer networks, specifically to the field of Local Area Network (LAN) signal reception and qualification.
Data receivers used in LAN applications must qualify received signals; that is, discriminate between valid data signals and spurious noise. This function is commonly known as receive squelch. One typical standard for receive squelch is the IEEE 802.3 standard (hereby incorporated by reference in its entirety). Section 14 of this, for Ethernet over twisted pair wiring, requires that all signals not meeting minimum amplitude and frequency requirements be filtered out. Implementing the amplitude requirements is relatively straightforward, but implementing the frequency requirements is more complex.
The IEEE 802.3 standard has different specifications for different transmission media. A 10Base-T specification is illustrated in FIG. 1. The 10base-T interface is an alternative to the more standard Attachment Unit Interface ("AUI"), which is also discussed in the 802.3 specification and which shares a number of the requirements discussed here. Referring to FIG. 1, an Ethernet controller 10 is coupled through a Manchester encoder 20 and a Manchester decoder 30 to a Medium Attachment unit ("MAU") 40. MAU 40 receives data from twisted pair line 50 and transmits data on twisted pair line 60, in communication with another MAU at the other end of the twisted pair. The data is Manchester encoded prior to transmission in order to provide clocking information within the data stream.
The 10Base-T specification requires that receivers squelch out single-cycle signals and all signals below 2 MHz in frequency. Once the receiver has begun accepting data, it must accept all signals within the frequency range of 5 and 10 MHz, while tolerating edge "jitter" (referring to the allowable phase variation of a signal's zero crossing, either forward or backward in time) of up to .+-.13.5 ns. The receiver must "shut off" (terminate data reception) if no signal transitions, either rising or falling, are detected for 230 ns, indicating the end of a data packet.
A MAU receiver must also recognize another type of signal, distinct from data, namely, a "Link Pulse". A Link Pulse is an idle signal exchanged by MAUs over the twisted pair segment as part of a "Link Integrity Test" function. The Link Pulse verifies that the receive pair of one MAU is functionally connected to the transmit pair of another MAU, and vice versa. Every 16 ms (.+-.8 ms) the MAU transmitter sends out a single 100 ns wide logic "1" (RD+ positive with respect to RD-) pulse. The MAU receiver at the other end of the segment must recognize this pulse as a valid Link Pulse, even though transmission-line effects may cause the pulse to "smear" out to 200 ns wide.
Conventional implementations of these frequency requirements have typically employed linear timing components, such as monostable multivibrators ("monostables"), based on resistive-capacitive time constants. For example, for data packet termination, the monostable would be triggered by edge transitions, and if no transition were received before it timed out, the receiver would treat the data packet as terminated and would shut off.
The conventional approach has two significant drawbacks: cost and accuracy. Space, and therefore cost, requirements of LAN adapters are becoming increasingly stringent, forcing them to be implemented with all of the necessary functions integrated together on a single ASIC (Application Specific Integrated Circuit). Linear functions, however, are expensive to integrate within an ASIC. Furthermore, linear functions such as monostables are not very accurate over the typical variations in temperature, voltage, and process conditions. Sufficient and consistent accuracy in the monostable to accept all signals having edge transitions occurring up to 127 ns apart, while rejecting those having edge transitions 230 ns apart, adds significant expense to the manufacturing environment for LAN equipment production.